1. Field of the Invention
The present invention relates to a method and an apparatus for reducing NOx emissions in CFB reactors used for combustion of fuel at atmospheric pressure.
2. Description of the Related Art
Typically, CFB reactors, circulating fluidized bed reactors, include a combustion chamber, a particle separator section with one or more particle separators and a convection section. A fast fluidized bed of particles is maintained in the combustion chamber, whereby flue gases and solid particles entrained therein are discharged from the upper part of the combustion chamber, the flue gases thereby being introduced into a first particle separator, which is typically a cyclone separator, in the particle separator section. There may be more than one particle separator arranged in the particle separator section between the combustion chamber and the convection section. Cleaned flue gases are discharged from the last particle separator in the particle separator section into a clean gas duct which connects the convection section to the last particle separator. Cooling surfaces are disposed in a cooling stage in the convection section.
In a typical CFB combustion process fuel, such as biofuel or brown coal, is introduced into the combustion chamber and burned at atmospheric pressure and at 700-1000.degree. C. Gases generated in the combustion process and solid bed particles are transported as a fast fluidized bed into the upper part of the combustion chamber and discharged therefrom into a particle separator section. Solid particles are separated from the flue gases in the particle separator section and at least partly returned to the combustion chamber through a return duct. The cleaned flue gases are discharged from the last particle separator into the clean gas duct and transported therethrough to the convection section, where the cleaned flue gases are cooled in a cooling stage.
An ongoing concern is the reduction of nitrogen oxide (NOx) emissions from the exhaust or flue gases before they are released into the atmosphere, since the NOx emissions are related to various environmental problems. It is evident that nitrogen oxide emissions result from any combustion reaction where air is present and/or the fuel used contains nitrogen. Nitrogen oxides are generated during combustion of fuels as a result of thermal fixation of nitrogen in the air and the conversion of fuel nitrogen. The former reaction is favored at high temperatures (above about 950.degree. C.) while the latter is of greater concern at lower temperatures, e.g., those generally found in fluidized bed combustion systems and which actually have been found to be beneficial in reducing nitrogen oxide emissions.
It is generally known that introduction of a reducing agent, such as ammonia or an ammonia precursor, into the flue gases may be used for reduction of nitrogen oxide (NOx) levels in flue gases.
Different solutions in this respect for reducing nitrogen oxides in flue gases from conventional furnaces, as well as, fluidized bed combustors have been suggested. U.S. Pat. No. 3,900,554 suggests removal of nitrogen oxides from flue gases which have exited a conventional furnace, by injecting ammonia (NH.sub.3) into the effluent stream. In order to provide favorable conditions for reducing NO.sub.x emissions in circulating fluidized bed boilers (CFB), several locations for ammonia injection have been suggested. U.S. Pat. Nos. 4,181,705 and 4,648,331 suggest injection of ammonia into the furnace, U.S. Pat. Nos. 5,462,718 and 5,538,704 suggest injection into the conduit between the furnace and a particle separator, and U.S. Pat. No. 4,756,890 and international publication WO91/07219 suggest injection into the particle separator.
U.S. Pat. No. 5,407,649, relating to a pressurized combustion process, discloses injecting ammonia or another nitrogen containing substance through a large number of nozzles into the flue gas flow at a particular location, where the flue gas flow still contains enough particles to build up a layer of solid particles on a filter surface arranged downstream of a first particle separator. It is suggested to inject ammonia immediately after a first coarse particle separation stage, at a location which ensures a long enough dwelling time for the flue gas and ammonia mixture before it reaches the filter surface and a final fine separation stage. The presented system requires a large number of injection nozzles, which may be located in a region extending from the freeboard in the furnace to the filter. The system functions well at high pressures, but at atmospheric pressure and at a temperature of about 800.degree. C. it gives only a slight NOx conversion rate and below 800.degree. C. it may even give a negative result, due to NOx being formed of NH.sub.3.
In the Proceedings, pages 725-730, of the Fluidized Bed Combustion Conference, ASME 1991, the effectiveness of different ammonia injection locations is compared. In measurements made at the top of the combustion chamber, the freeboard was found to provide the optimum position for injecting ammonia under different operating conditions and for different types of fuels. Also, the outlet of the first coarse particle separating cyclone was tested as an ammonia injection location, but it seemed to lead to somewhat higher NOx emissions than when injecting ammonia into the top of the freeboard.
Contrary to the findings of the last mentioned study, it has now been noticed that the optimum location for ammonia injection varies depending on such factors as the load conditions of the boiler and the fuel type. In many combustion processes, the use of some of the locations suggested in previous patent publications and the above study leads to a satisfactory NOx reduction. However, in highest load conditions, or when using fuel including a large amount of volatile components, such as biofuel or brown coal, the NOx reducing schemes described above do not seem to operate as required. For such situations, new schemes for feeding NOx reducing agent are required.
Many of the presently used schemes for injecting NOx reducing agent are such that a large number of feeding points are required to provide sufficient distribution of the reducing agent. The feeding nozzles as such may, however, cause various problems, as nozzles are prone to erosion and blocking caused by solid material in the flue gas stream, and may, therefore, not function as required. Also, when assembling the nozzles within a centrifugal separator, the mounting may require separate additional constructions within the separator, which constructions may easily hinder the gas flow and thus degrade the separation efficiency of the separator.